Your app declares support for external-accessory in the UIBackgroundModes key in your Info.plist file but does not declare any external accessory protocols supported by your app. The external accessory background mode is intended for apps that communicate with hardware accessories through the External Accessory framework.
If your app is meant to work with external hardware, supported protocols must be included in the UISupportedExternalAccessoryProtocols key in your app's Info.plist file - and the hardware's PPID # should be provided in the Review Notes field of your app in iTunes Connect.
Additionally, your app must be authorized by MFi to use the desired hardware. If you are not yet in the MFi Program, you can enroll at MFi program.
Please either revise your Info.plist to include the UISupportedExternalAccessoryProtocols key and update your Review Notes to include the PPID # - or remove the external-accessory value from the UIBackgroundModes key.